In comparison with cable transmission, optical transmission has such advantages as greater capacity, less loss and greater power for avoiding electromagnetic interference. Therefore, along with the gradual descent of optical transmission cost, the optical access network has become the inevitable developing trend. The Passive Optical Network (PON), which uses passive devices, is a most promising technology for implementing a wideband optical access network.
The PON technology is classified by the bearer content as ATM Based PON (APON), Ethernet Based PON (EPON), Gigabit PON (GPON), etc.
Referring to FIG. 1, a PON is usually composed of an Optical Line Terminal (OLT) residing in the Center Office (CO) and a plurality of Optical Network Units (ONUs) or Optical Network Terminals (ONTs) residing in the customer premise. An Optical Distribution Network (ODN) consisting of optical fiber, passive optical splitter or coupler is used to connect such devices of CO to the devices of the customer premise.
Referring to FIG. 2, the transfer of downlink data from the PON network to the customer premise is different from that of uplink data from the customer premise to the PON network. The downlink data is broadcasted from the OLT to each ONU. Each ONU matches destination address carried in protocol transmission unit header of the data received with the address of the ONU, and processes the data if the destination address carried in the data matches the ONU's address.
Due to media sharing characteristic of ODN, the uplink data transmission is relatively complex. To avoid collision, the uplink data is transmitted using Time Division Multiple Address (TDMA) mode, and the uplink data transmission are controlled by the OLT according to control mechanism of the OLT.
In a point-to-multipoint passive optical network system, the uplink data is transmitted using the TDMA mode. Each ONU or ONT sends data to the OLT in the TDMA mode. In normal conditions, the OLT assigns timeslot for each ONU (i.e. authorizes each ONU) to guarantee only one ONU lights at one period of time. The ONU opens a light module only within the timeslot (authorization) assigned by the OLT, which makes no collision.
However, in the case of the above method, if an ONU fails and sends uplink data at a period time rather than the timeslot assigned by the OLT, the uplink data sent by this ONU will overlap the uplink data of other adjacent ONUs, which makes data errors.
An uplink data frame includes two parts, i.e. frame header and data payload, as shown in FIG. 3. The frame header and the data payload can be distinguished by a frame delimiter therebetween. The frame header is mainly used for frame synchronization. In TDMA uplink communications, an OLT takes frame delimiter detected time as the frame reaching time of the uplink data frame.
When an ONU in an optical network fails, e.g. ONU1 in FIG. 4 fails, ONU1 may transmit an uplink non-framing signal, e.g. signal not in frame or meaningless signal, earlier or later, and this non-framing signal may overlap the uplink data frame of ONU2. Since the signal sent by ONU1 is a non-framing signal, the OLT is unable to detect the coming of the non-framing signal from ONU1 and regards that the uplink data frame from ONU1 is lost. Furthermore, since the non-framing signal of ONU1 overlaps part of the frame of ONU2, the OLT may be unable to detect the frame delimiter and can not identify where the frame header of ONU2 is located. Thus the LT may regard that the data frame from ONU2 is also lost.
When the above case occurs, since the OLT regards that data frames from both ONU1 and ONU2 are lost, it is difficult to determine which ONU fails.